Towards new design rules for indoor photovoltaic cells
The common design rules for outdoor solar cells do not apply necessarily for indoor photovoltaic cells due to differences in spectra and intensities. Since no general standard for indoor conditions is currently available, the first aim of this paper is to present a statistical approach encompassing the diversity of available intensities and spectra which shall enable a systematic derivation of a standard for indoor conditions. In a second step, the influences of injection-and spectra-dependent parameters on cell efficiencies are studied with the focus on indoor conditions. It is shown that the parallel resistance has to be increased above 50 k ohm cm2 in order to achieve an efficiency above 10% at intensities below 10-3 W/cm2 (~0.01 suns). The influence of the series resistance is negligible in this intensity range, allowing values in the order of 10 ohm cm2 without appreciable efficiency reduction. This gives room for wider design variations with respect to base and emitter doping, optimization of bulk-, surface- and emitter recombination, contact optimization and back contact cell designs. The combined evaluation of possible photovoltaic cell parameters show that efficiencies of more than 10% at intensities as low as 10-4 W/cm2 are accessible.